Islamist rebels chased Mali's army from a garrison town deep in its own territory on Monday, striking back at the weakest link in a nascent coalition after French fighter jets hit militant bases deep in the Sahara.

The surprise move by what witnesses called a well-armed rebel force highlights the risk that the French campaign in Mali could widen as al Qaeda militants spread across the heart of the world's largest desert.

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French soldiers leave a hangar at a Malian air base in Bamako Monday. France intervened as militants in the north began to expand territory.
Reuters

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The advancing fighters took control of the small barracks town of Diabaly after attacking and defeating the Malian army there, French Defense Minister Jean-Yves Le Drian said.

The once-remote conflict in northern Mali is now within 250 miles of Mali's capital, Bamako, posing a grave test for the country's ramshackle military and adding to the mounting pressure for other foreign forces to get involved.

Mr. Fabius said the U.S. was helping with transport, communication and intelligence-gathering capabilities, while the U.K., Belgium and Denmark were providing transport assistance. Several West African countries, including Nigeria, Niger, Burkina Faso and Senegal, have pledged to dispatch hundreds of soldiers, but the timetable for their deployment has yet to be determined. Algeria, which has traditionally opposed intervention, allowed French combat aircraft to fly through its airspace to reach targets in Mali on Sunday.

The European Union said it would accelerate the deployment of its own mission to train Malian soldiers, but ruled out any potential combat engagement.

United Nations Security Council members, meeting Monday, didn't object to the French intervention, French Ambassador Gérard Araud said, portraying the response as an expression of support for the campaign.

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France says its action is covered by a December Security Council resolution authorizing a military intervention by the African Union.

The French air campaign, which began on Friday, aims to uproot the Islamist militant groups—most prominent among them al Qaeda in the Islamic Maghreb, or AQIM—from their strongholds in northern Mali.

The militants, however, have fallen back on a strength: extreme mobility.

"These guys would break camp on five minutes' notice," said Canadian diplomat Robert Fowler, who was held hostage by AQIM for five months in 2009.

AQIM, operating across a vast stretch of the Sahara, has furnished arms, training and recruits to Mali-based rebel outfits bent on imposing strict Islamic law as widely as possible across West Africa.

Rebels on Monday advanced south from rebel-held northern Mali to Diabaly, about 30 miles into government-controlled territory.

France launched air attacks on Islamist rebels in Mali to prevent their advance into the south of the country. WSJ's Drew Hinshaw reports from Bamako, Mali. Photo: Getty Images

"They were extremely well-armed—rocket launchers, everything," said local teacher Pierre Kone, who described a 30-truck convoy entering the town.

A spokesman for one of Mali's larger Islamist militias said his fighters raided Diabaly not to prepare an assault on the capital—but to throw Mali's military off-balance. "Our strategy is very clear," he said. "We're going to lead our own war….They won't decide what kind of war we fight."

People in Diabaly were fleeing—and encountering the rebels. "They came in to introduce themselves and say that they're here to defend Islam, and to fight the Malian army," said Mr. Kone, the teacher.

The Malian army has sent reinforcements to retake the town, according to Mali Defense Ministry spokesman Nouhoum Togo.

The attack on Diabaly came shortly after French and Malian forces said they had regained the small central Mali town of Konna, which had fallen into rebel hands last week. Territory there includes an airfield that French and Malian officials described as vital to any liberation campaign.

Over the weekend, fighter jets from France attacked targets deep in territories controlled by Islamist groups, notably the large city of Gao. The French also targeted sites along the border with Mauritania, as well as Kidal, a remote trading post near Algeria. The extent of casualties wasn't yet clear.

The Islamist rebellion grew stronger with the end of the conflict in Libya in 2011, as militants and looted weapons from Libya made their way to Mali.

Heavily armed insurgents then put pressure on the Malian army, which in turn lost control of the country's northern regions. In March 2012, the army mutinied and junior officers seized control of the government.

Corrections & Amplifications Al Qaeda in the Islamic Maghreb, or AQIM uses Mali's desert north, a region the size of France, as a base for operations across a much larger stretch of the Sahara. A previous version of this article incorrectly said AQIM operates across a stretch of the Sahara the size of France.

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